For restorative treatment of tooth structures (enamel, dentin, and cementum) damaged, for example, by dental caries, dental cements are used as materials for luting dental prostheses such as crowns, inlays, and bridges to broken or chipped tooth crowns. A dental cement is usually composed of a polymerizable monomer, a filler, and a polymerization initiator. (Meth)acrylate-based polymerizable monomers are widely used as such polymerizable monomers.
It is desirable that a dental cement have high adhesiveness to tooth structures (in particular to dentin) in order to prevent detachment of a prosthesis after restorative treatment and to prevent secondary caries. For improvement of the adhesiveness to dentin, it is considered important to allow a polymerizable monomer component contained in the dental cement to penetrate into the collagen layer of dentin and to cure therein so as to form a dentin-dental cement hybrid layer (a so-called resin-impregnated layer). The use of a hydrophilic multifunctional (meth)acrylate-based polymerizable monomer having a specific chemical structure as such a polymerizable monomer has been proposed to improve the adhesiveness of the dental cement to dentin.
On the other hand, (meth)acrylate-based polymerizable monomers have the disadvantage of being susceptible to hydrolysis during storage and thus having low storage stability. Therefore, dental materials containing multifunctional (meth)acrylamide-based polymerizable monomers have been proposed to provide dental materials having high resistance to hydrolysis.
Examples of such conventional dental materials are as follows. Patent Literature 1 proposes a composition containing a (meth)acrylate-based polymerizable monomer having at least two polymerizable groups and at least two primary hydroxyl groups, as a composition suitable for use as a dental composition (including a dental cement). Patent Literature 2 proposes a self-adhesive dental cement containing a (meth)acrylate-based polymerizable monomer having an unconjugated carbon chain with at least four singly-bonded carbon atoms, at least two polymerizable groups, and at least two hydroxyl groups.
Patent Literature 3 proposes a dental material (including a dental cement) containing two (meth)acrylamide-based polymerizable monomers: a bifunctional (meth)acrylamide-based polymerizable monomer represented by the general formula (3) having two (meth)acrylamide groups both of which are secondary amide groups; and a (meth)acrylamide-based polymerizable monomer represented by the general formula (4) having two (meth)acrylamide groups both of which are tertiary amide groups (hereinafter, in the present description, a (meth)acrylamide-based polymerizable monomer having two (meth)acrylamide groups both of which are secondary amide groups and a (meth)acrylamide-based polymerizable monomer having two (meth)acrylamide groups both of which are tertiary amide groups may be referred to as symmetric (meth)acrylamide compounds, for the sake of convenience).
where Ra, Rb, Rc, and Rd are each independently a hydrogen atom or a methyl group, Re and Rf are each independently a group other than a hydrogen atom, such as an alkyl group or an aryl group, and Xa and Xb are each independently a divalent organic group optionally having an oxygen atom and a nitrogen atom.
However, the hydrophilic multifuctional (meth)acrylate-based polymerizable monomers disclosed in Patent Literatures 1 and 2 and the bifunctional (meth)acrylamide-based polymerizable monomer represented by the general formula (3) disclosed in Patent Literature 3 have the following disadvantages. Most of these polymerizable monomers are crystalline solids and must be used in combination with a large amount of a hydrophilic monofunctional (meth)acrylate-based polymerizable monomer such as 2-hydroxyethyl (meth)acrylate to obtain a homogeneous composition, and thus only a limited range of compositions can be prepared. In addition, when any of these polymerizable monomers are used in a dental cement, the resulting cured product has high water absorbency and low mechanical strength. The (meth)acrylamide-based polymerizable monomer represented by the general formula (4) is oily in nature and has good compatibility with other polymerizable monomers, but due to its low hydrophilicity, a dental cement containing this oily compound has the disadvantage of low adhesiveness to tooth structures.